Method and System for Implementing Network Element-Level Redundancy

ABSTRACT

According to an additional embodiment, a method may include communicatively coupling a first network element to a second network element via a first path of a first point-to-point network. The method may also include communicatively coupling the first network element to a third network element via a second path of the first-point-to-point network. The method may additionally include communicatively coupling the second network element to a fourth network element via a first path of a second point-to-point network. The method may further include communicatively coupling the third network element to the fourth network element via a second path of the second point-to-point network. The method may also include configuring the first path and the second path of the first point-to-point network as paths of a first linear protected switching connection and the first path and the second path of the second point-to-point network as paths of a second linear protected switching connection such that traffic associated with a service and communicated between the first network element and the fourth network element via the first path of the first point-to-point network and the first path of the second point-to-point network may be switched over to the second path of the first point-to-point network and the second path of the second point-to-point network in response to an event.

RELATED APPLICATION

This application is related to copending Patent Application entitled “Method and System for Implementing Network Element-Level Redundancy,” application Ser. No. ______ (064731.0803), filed on the same date as the present application.

This application is also related to copending Patent Application entitled “Method and System for Implementing Network Element-Level Redundancy,” application Ser. No. ______ (064731.0804), filed on the same date as the present application.

This application is also related to copending Patent Application entitled “Method and System for Implementing Network Element-Level Redundancy,” application Ser. No. (064731.0806), filed on the same date as the present application.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates generally to networked communications and, more particularly, to a method and system for implementing network element-level redundancy.

BACKGROUND

In telecommunications, information is often sent, received, and processed according to the Open System Interconnection Reference Model (OSI Reference Model or OSI Model). In its most basic form, the OSI Model divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers, which are also known respectively as Layer 7 (L7), Layer 6 (L6), Layer 5 (L5), Layer 4 (L4), Layer 3 (L3), Layer 2 (L2), and Layer 1 (L1). It is therefore often referred to as the OSI Seven Layer Model.

Layer 2 is the layer which typically transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment. Layer 2 provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in Layer 1. Examples of Layer 2 protocols are Ethernet for local area networks (multi-node), the Point-to-Point Protocol (PPP), HDLC and ADCCP for point-to-point (dual-node) connections. Layer 2 data transfer may be handled by devices known as switches.

To ensure high reliability and availability in communications networks, protection switching is often used. When implemented, protection switching typically provides a primary or “working” path for a network and a redundant or “protection” path for the network. Accordingly, each path may be monitored, and if a failure is detected on the working path, network traffic may be switched to the protection path. An example of protection switching may be Ethernet Linear Protection Switching (ELPS) as defined by the ITU G.8031 standard.

While protection switching may provide redundancy for link or path failures, it does not provide redundancy in the event of a failure of a network element (e.g., a switch) redundantly-interfaced to a point-to-point network. Accordingly, this disclosure provides for such network-element level redundancy.

SUMMARY

In accordance with the present disclosure, disadvantages and problems associated with creating redundancy in L2 networks may be reduced or eliminated.

According to one embodiment, a method may include communicatively coupling a first network element to a second network element via a first path of a point-to-point connection. The method may also include communicatively coupling the first network element to a third network element via a second path of the point-to-point network. The method may further include configuring the first path and the second path as paths of a linear protected switching connection (LPSC) such that traffic associated with a service and communicated via one of the first path and the second path may be switched over to the other of the first path and the second path in response to an event.

According to another embodiment, a method may include communicatively coupling a first network element to a second network element via a first link of a multi-chassis link aggregation group. The method may also include communicatively coupling the first network element to a third network element via a second link of the multi-chassis link aggregation group. The method may additionally include communicatively coupling the second network element to a fourth network element via a first path of a point-to-point network. The method may further include communicatively coupling the third network element to the fourth network element via a second path of the point-to-point network. The method may also include configuring the first path and the second path as paths of a linear protected switching connection such that traffic associated with a service and communicated between the first network element and the fourth network element via the first link and the first path may be switched over to the second link and the second path in response to an event.

According to an additional embodiment, a method may include communicatively coupling a first network element to a second network element via a first path of a first point-to-point network. The method may also include communicatively coupling the first network element to a third network element via a second path of the first-point-to-point network. The method may additionally include communicatively coupling the second network element to a fourth network element via a first path of a second point-to-point network. The method may further include communicatively coupling the third network element to the fourth network element via a second path of the second point-to-point network. The method may also include configuring the first path and the second path of the first point-to-point network as paths of a first linear protected switching connection and the first path and the second path of the second point-to-point network as paths of a second linear protected switching connection such that traffic associated with a service and communicated between the first network element and the fourth network element via the first path of the first point-to-point network and the first path of the second point-to-point network may be switched over to the second path of the first point-to-point network and the second path of the second point-to-point network in response to an event.

According to a further embodiment, a method may include communicatively coupling a first network element to a second network element via a first path of a point-to-point network. The method may also include communicatively coupling the first network element to a third network element via a second path of the point-to-point network. The method may additionally include communicatively coupling the second network element and the third network element to a multipoint-to-multipoint network. The method may further include configuring the first path and the second path as paths of a linear protected switching connection such that traffic associated with a service and communicated between the first network element and the multipoint-to-multipoint network via one of the first path and the second path may be switched over to the other of the first path and the second path in response to an event.

Certain embodiments of the disclosure may provide one or more technical advantages. A technical advantage may be that a network may provide network element-level redundancy for point-to-point Ethernet virtual channels across a network.

Certain embodiments of the disclosure may include none, some, or all of the above technical advantages. One or more other technical advantages may be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of an example network including protection switching among multiple network elements, in accordance with certain embodiments of the present disclosure;

FIG. 2 illustrates a block diagram an example network element, in accordance with certain embodiments of the present disclosure;

FIG. 3 illustrates a block diagram of an example network including protection switching among multiple network elements, wherein such multiple network elements are interfaced to a single link aggregation group, in accordance with certain embodiments of the present disclosure;

FIG. 4 illustrates a block diagram of an example network including protection switching among multiple network elements interfaced between point-to-point network domains, in accordance with certain embodiments of the present disclosure;

FIG. 5 illustrates a block diagram of another example network including protection switching among multiple network elements interfaced between point-to-point network domains, in accordance with certain embodiments of the present disclosure; and

FIG. 6 illustrates a block diagram of an example network including protection switching among multiple network elements interfaced between a point-to-point network and a multipoint-to-multipoint network, in accordance with certain embodiments of the present disclosure.

DETAILED DESCRIPTION

FIG. 1 illustrates a block diagram of an example network 100 a including protection switching among multiple network elements 102 (e.g., network elements 102 b and 102 c), in accordance with certain embodiments of the present disclosure. As shown in FIG. 1, network 100 a may include network element 102 a communicatively coupled to network element 102 b and network element 102 c via point-to-point network 122.

Each network element 102 (e.g., network elements 102 a-102 c) in network 100 a and network elements 102 described in FIG. 3-6 (e.g., network elements 102 d-102 s of networks 100 b-110 e) may comprise any suitable system operable to transmit and receive traffic. As used herein, “traffic” means information transmitted, stored, or sorted in a network 100 (e.g., networks 100 a-100 e). Such traffic may comprise optical or electrical signals configured to encode audio, video, textual, and/or any other suitable data. The data may also be real-time or non-real-time. Traffic may be communicated via any suitable communications protocol, including, without limitation, the Open Systems Interconnection (OSI) standard and Internet Protocol (IP). Additionally, the traffic communicated in networks 100 may be structured in any appropriate manner including, but not limited to, being structured in frames, packets, or an unstructured bit stream. In the illustrated embodiment, each network element 102 may be operable to transmit traffic to one or more other network elements 102 and receive traffic from the one or more other network elements 102. Network elements 102 will be discussed in more detail below with respect to FIG. 2.

Point-to-point network 122 may be any network of one or more network elements (e.g., routers and switches) suitable to provide one or more point-to-point paths between network element 102 a and network element 102 b, and between network element 102 a and network element 102 c.

Network element 102 a may communicate with network elements 102 b and 102 c using linear protected switching. Accordingly, network element 102 a may be communicatively coupled to network elements 102 b and 102 c through a linearly protected switching connection. The linearly protected switching connection may comprise a working path 118 and a protection path 120. Network element 102 a may be communicatively coupled to network element 102 b via working path 118, and may be communicatively coupled to network element 102 c via protection path 120. In certain embodiments, with respect to network element 102 a, the linearly protected switching connection may be configured in accordance with the ITU G.8031 standard. In these and other embodiments, with respect to network elements 102 b and 102 c, the linear protected switching connection may be configured in connection with a modified multi-chassis version of the ITU G.8031 standard. In addition, network element 102 a may be communicatively coupled to other network entities (e.g., other network elements) via link 126 a, network element 102 b may be communicatively coupled to other network entities via link 126 b, and network element 102 c may be communicatively coupled to other network entities via link 126 c.

Network element 102 a may be configured to perform linear protected switching between working path 118 and protection path 120. For example, network element 102 a may be configured to perform protection switching between paths 118 and 120 in accordance with the G.8031 standard and may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching. In certain embodiments, network element 102 a may be multi-homed such that network elements 102 b and 102 c appear as a single network element to network element 102 a.

In addition, network elements 102 b and 102 c may be configured to implement protection switching between working path 118 and protection path 120. For example, network elements 102 b and 102 c may be configured to perform protection switching between paths 118 and 120 in accordance with the G.8031 standard and each may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching. In addition, network elements 102 b and 102 c may be communicatively coupled via a synchronization connection 124. Via synchronization connection 124, network elements 102 b and 102 c may communicate to each other such state information and switchover information. In certain embodiments, such communication via synchronization connection 124 may be in accordance with Inter-Control Center Communications Protocol (ICCP) or similar protocol. Synchronization connection 124 may include a direct link, a point-to-point connection over point-to-point network 122, a dedicated management network, or any other suitable type of connection. In these and other embodiments, synchronization connection 124 may include redundancy (e.g., multiple physical links between network elements 102 b and 102 c) to provide high reliability and availability of communication between network elements 102 b and 102 c.

In operation, paths 118 and 120 may provide connectivity for services between network element 102 a and network elements 102 b and 102 c. For a particular service, one of paths 118, 120 may be designated as active, such that traffic for such service is communicated over such active path. In the event of a failure or other event, one or more of network elements 102 a, 102 b, and 102 c may cause a protection switch of the service from the active path to the other path (e.g., from working path 118 to protection path 120). In certain embodiments, such protection switching may be implemented in accordance with the G.8031 standard.

In addition, during operation paths 118 and 120 may be configured in accordance with any suitable redundancy scheme. For example, a 1+1 protection scheme may be used in which traffic is replicated among paths 118 and 120 and an end point network element may determine which path to select. As another example, a 1:1 scheme may be used in which all traffic is communicated over a single path (e.g., working path 118) and is protection switched to the other (e.g., protection path 120) in response to an event.

Protection switching from one path to another may occur in response to any suitable event. For example, failure of a path 118, 120 may cause a switchover to the other path. As another example, failure of a link 126 b or 126 c or other event upstream of network elements 102 b and/or 102 c may cause a switchover (e.g., failure of link 126 b may cause a switchover from working path 118 to protection path 120). As an additional example, a switchover may occur automatically after a failure condition has been remedied (e.g., a wait to restore timer initiated after a failure on working path 118 may trigger a switchover from protection path 120 to working path 118). As a further example, a switchover may occur in response to a human-initiated action (e.g., a command to switchover issued by a network operator/administrator). All such events may be monitored by network elements 102 b and 102 c and all such monitoring may be synchronized between network elements 102 b and 102 c via synchronization connection 124. In addition, switchovers may be initiated by network element 102 b and/or network element 102 c in response to such monitored events, and such initiation of such switchovers may also be synchronized between network elements 102 b and 102 c via synchronization connection 124.

A technical advantage of network 100 a is that it provides for network element-level redundancy for point-to-point Ethernet virtual channels across a network. In addition, an access network element (e.g., network element 102 a) need not be directly connected to network elements 102 b and 102 c, as would be the case with multiple chassis link aggregation.

FIG. 2 illustrates a block diagram an example network element 102, in accordance with certain embodiments of the present disclosure. Network element 102 of FIG. 2 may be exemplary of the various network elements discussed elsewhere in this disclosure (e.g., network elements 102 a-102 p). Each network element 102 may be coupled to one or more other network elements 102 via one or more transmission media 12. Each network element 102 may generally be configured to receive data from and/or transmit data to one or more other network elements 102. In certain embodiments, network element 102 may comprise a switch configured to route data received by network element 102 to another device (e.g., another network element 102) coupled to network element 102.

As depicted in FIG. 2, each network element 102 may include a master control unit 103, a switching element 104, and one or more network interfaces 106 communicatively coupled to each of master control unit 103 and switching element 104.

Master control unit 103 may include any suitable system, apparatus, or device configured to manage network element 102, including management of routing of data between ports 110. As shown in FIG. 2, master control unit 103 may maintain a routing table, wherein such routing table may include any table, database, file, or other data structure configured to maintain information relating a particular ingress port 110 and/or link aggregation group (LAG) 112 to a corresponding egress port 110 and/or LAG 112.

Switching element 104 may be communicatively coupled to master control unit 103 and may include any suitable system, apparatus, or device configured to receive traffic via a port 110 and route such traffic to a particular network interface 106 and/or port 110 based on analyzing the contents of the data and/or based on a characteristic of a signal carrying the data (e.g., a wavelength and/or modulation of the signal). For example, in certain embodiments, a switching element 104 may include a switch fabric (SWF).

Each network interface 106 may include any suitable system, apparatus, or device configured to serve as an interface between a network element 102 and a transmission medium 12. Each network interface 106 may enable its associated network element 102 to communicate to other network elements 102 using any suitable transmission protocol and/or standard. Network interface 106 and its various components may be implemented using hardware, software, or any combination thereof. For example, in certain embodiments, one or more network interfaces 106 may include a network interface card. In the same or alternative embodiments, one or more network interfaces 106 may include a line card.

As depicted in FIG. 2, each of network interfaces 106 may include one or more physical ports 110. Each physical port 110 may include any system, device or apparatus configured to serve as a physical interface between a corresponding transmission medium 12 and network interface 106. For example, a physical port may comprise an Ethernet port, an optical port, or any other suitable port.

As shown in FIG. 2, two or more physical ports 110 of a particular network element 102, their corresponding physical ports 110 of another network element 102, and their corresponding transmission media 12 may be grouped into a link aggregation group (LAG) 112. Although each LAG 112 in FIG. 2 is depicted as including a particular number of member physical ports 110, LAG 112 may include any suitable number of member physical ports 110. LAG 112 may combine its member ports or member LAGs using link aggregation such that the member ports are represented as a single logical port to components of a network 100 (e.g., a network 100 a-100 e) external to LAG 112. Although each LAG 112 in FIG. 2 is depicted as including only ports 110 of a single network element 102, a LAG 112 may in some embodiments include ports 110 of two or more network elements 102 (e.g., multi-chassis link aggregation).

FIG. 3 illustrates a block diagram of an example network 100 b including protection switching among multiple network elements (e.g., network elements 102 e and 102 f), wherein such multiple network elements are interfaced to a single multi-chassis link aggregation group (e.g., LAG 112), in accordance with certain embodiments of the present disclosure. As shown in FIG. 3, network 100 b may include network element 102 d communicatively coupled to network element 102 e and network element 102 f via a multi-chassis LAG 112, and network element 102 g communicatively coupled to network element 102 e and network element 102 f via point-to-point network 122.

As described above, each of network elements 102 d-102 g may comprise any suitable system operable to transmit and receive traffic. Point-to-point network 122 may be any network of one or more network elements (e.g., routers and switches) suitable to provide one or more point-to-point paths between network element 102 e and network element 102 g, and between network element 102 f and network element 102 g. Point-to-point network 122 depicted in FIG. 3 may be similar to point-to-point network 122 depicted in FIG. 1.

Network element 102 d may be multi-homed via LAG 112 to network elements 102 e and 102 f such that network elements 102 e and 102 f appear as a single network element to network element 102 d.

Network element 102 g may communicate with network elements 102 e and 102 f using linear protected switching. Accordingly, network element 102 g may be communicatively coupled to network elements 102 e and 102 f through a linearly protected switching connection. The linearly protected switching connection may comprise a working path 118 and a protection path 120 and may be similar to the linearly protected switching connection described above with respect to FIG. 1. Network element 102 g may be configured to perform linear protected switching between working path 118 and protection path 120. For example, network element 102 g may be configured to perform protection switching between paths 118 and 120 in accordance with the G.8031 standard and may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching.

In addition, network elements 102 e and 102 f may be configured to implement protection switching between working path 118 and protection path 120. For example, network elements 102 e and 102 f may be configured to perform protection switching between paths 118 and 120 in accordance with a modified multi-chassis version of the G.8031 standard and each may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching. In addition, network elements 102 e and 102 f may be communicatively coupled via a synchronization connection 124. Via synchronization connection 124, network elements 102 e and 102 f may communicate to each other such state information and switchover information. Synchronization connection 124 depicted in FIG. 3 may be similar to synchronization connection 124 depicted in FIG. 1.

In operation, paths 118 and 120 may provide connectivity for services between network element 102 g and network elements 102 e and 102 f (and ultimately, connectivity for services between network elements 102 d and 102 g). For a particular service, one of paths 118, 120 may be designated as active, such that traffic for such service is communicated over such active path. In the event of a failure or other event, one or more of network elements 102 e, 102 f, and 102 g may cause a protection switch of the service from the active path to the other path (e.g., from working path 118 to protection path 120). In certain embodiments, such protection switching may be implemented in accordance with the G.8031 standard.

Similarly, for a particular service, one of member links 128 a and 128 b of LAG 112 may be designated as active and the other as standby, such that traffic for such service is communicated over such active link. In the event of a failure or other event, one or more of network elements 102 d, 102 e, and 102 f may cause traffic associated from the service to be switched from the active link to the standby link (e.g., from link 128 a to link 128 b).

In addition, during operation paths 118 and 120 and links 128 may be configured in accordance with any suitable redundancy scheme. For example, a 1+1 protection scheme may be used in which traffic is replicated among paths 118 and 120 and links 128 and an end point network element may determine which path/link combination to select. As another example, a 1:1 scheme may be used in which all traffic is communicated over a single path/link combination (e.g., working path 118 and link 128 a) and is protection switched to the other (e.g., protection path 120 and link 128 b) in response to an event.

Protection switching from one path/link combination to another may occur in response to any suitable event. For example, failure of either of a path 118, 120 or a link 128 in a path/link combination may cause a switchover to the other path/link combination (e.g., a failure in either of path 118 or link 128 a may cause switchover to the combination of path 120 and link 128 b). As an additional example, a switchover may occur automatically after a failure condition has been remedied (e.g., a wait to restore timer initiated after a failure on working path 118 may trigger a switchover from protection path 120 to working path 118, and a switchover from link 128 b to link 128 a). As a further example, a switchover may occur in response to a human-initiated action (e.g., a command to switchover issued by a network operator/administrator). All such events may be monitored by network elements 102 e and 102 f and all such monitoring may be synchronized between network elements 102 e and 102 f via synchronization connection 124. In addition, switchovers may be initiated by network element 102 e and/or network element 102 f in response to such monitored events, and such initiation of such switchovers may also be synchronized between network elements 102 e and 102 f via synchronization connection 124.

A technical advantage of network 100 b is that it provides for network element-level redundancy for point-to-point Ethernet virtual channels across a network that is interoperable with multi-chassis link aggregation.

FIG. 4 illustrates a block diagram of an example network 100 c including protection switching among multiple network elements 102 i and 102 j interfaced between point-to-point network domains 122 a and 122 b, in accordance with certain embodiments of the present disclosure. As shown in FIG. 4, network 100 c may include network element 102 h communicatively coupled to network element 102 i and network element 102 j via point-to-point network 122 a, and network element 102 k communicatively coupled to network element 102 i and network element 102 j via point-to-point network 122 b.

As described above, each of network elements 102 h-102 k may comprise any suitable system operable to transmit and receive traffic. Point-to-point network 122 a may be any network of one or more network elements (e.g., routers and switches) suitable to provide one or more point-to-point paths between network element 102 h and network element 102 i, and between network element 102 h and network element 102 j. Point-to-point network 122 b may be any network of one or more network elements (e.g., routers and switches) suitable to provide one or more point-to-point paths between network element 102 k and network element 102 i, and between network element 102 k and network element 102 j. Point-to-point networks 122 a and 122 b depicted in FIG. 4 may be similar to point-to-point networks 122 depicted in FIGS. 1 and 3.

Network element 102 h may be multi-homed such that network elements 102 i and 102 j appear as a single network element to network element 102 h.

Network element 102 h may communicate with network elements 102 i and 102 j using linear protected switching. Accordingly, network element 102 h may be communicatively coupled to network elements 102 i and 102 j through a linearly protected switching connection. The linearly protected switching connection may comprise a working path 118 a and a protection path 120 a and may be similar to the linearly protected switching connections described above with respect to FIGS. 1 and 3. Network element 102 h may be configured to perform linear protected switching between working path 118 a and protection path 120 a. For example, network element 102 h may be configured to perform protection switching between paths 118 a and 120 a in accordance with the G.8031 standard and may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching.

Furthermore, network element 102 k may communicate with network elements 102 i and 102 j using linear protected switching. Accordingly, network element 102 k may be communicatively coupled to network elements 102 i and 102 j through a linearly protected switching connection. The linearly protected switching connection may comprise a working path 118 b and a protection path 120 b and may be similar to the linearly protected switching connections described above with respect to FIGS. 1 and 3. Network element 102 k may be configured to perform linear protected switching between working path 118 b and protection path 120 b. For example, network element 102 k may be configured to perform protection switching between paths 118 b and 120 b in accordance with the G.8031 standard and may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching.

In addition, network elements 102 i and 102 j may be configured to implement protection switching between working path 118 a and protection path 120 a and protection switching between working path 118 b and protection path 120 b. For example, network elements 102 i and 102 j may be configured to perform protection switching between paths 118 a and 120 a and between paths 118 b and 120 b in accordance with a modified multi-chassis version of the G.8031 standard and each may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching. In addition, network elements 102 i and 102 j may be communicatively coupled via a synchronization connection 124. Via synchronization connection 124, network elements 102 i and 102 j may communicate to each other such state information and switchover information. Synchronization connection 124 depicted in FIG. 4 may be similar to synchronization connections 124 depicted in FIGS. 1 and 3.

In operation, paths 118 a and 120 a may provide connectivity for services between network element 102 h and network elements 102 i and 102 j (and ultimately, connectivity for services between network elements 102 h and 102 k). For a particular service, one of paths 118 a, 120 a may be designated as active, such that traffic for such service is communicated over such active path. In the event of a failure or other event, one or more of network elements 102 h, 102 i, and 102 j may cause a protection switch of the service from the active path to the other path (e.g., from working path 118 a to protection path 120 a). In certain embodiments, such protection switching may be implemented in accordance with the G.8031 standard.

Similarly, paths 118 b and 120 b may provide connectivity for services between network element 102 k and network elements 102 i and 102 j (and ultimately, connectivity for services between network elements 102 h and 102 k). For a particular service, one of paths 118 b, 120 b may be designated as active, such that traffic for such service is communicated over such active path. In the event of a failure or other event, one or more of network elements 102 k, 102 i, and 102 j may cause a protection switch of the service from the active path to the other path (e.g., from working path 118 b to protection path 120 b). In certain embodiments, such protection switching may be implemented in accordance with the G.8031 standard.

In addition, during operation paths 118 a, 118 b, 120 a and 120 b may be configured in accordance with any suitable redundancy scheme. For example, a 1+1 protection scheme may be used in which traffic is replicated among paths 118 and 120 a, and an end point network element may determine which paths to select. As another example, a 1:1 scheme may be used in which all traffic is communicated over a single pair of paths (e.g., working paths 118 a and 118 b) and is protection switched to the other pair of paths (e.g., protection paths 120 a and 120 b) in response to an event.

Protection switching from one pair of paths to another may occur in response to any suitable event. For example, failure of any path 118 a, 118 b, 120 a, or 120 b may cause a switchover in both point-to-point networks 122 a and 122 b (e.g., a failure in either of path 118 a or path 118 b may cause switchover to path 120 a and 120 b). As an additional example, a switchover may occur automatically after a failure condition has been remedied (e.g., a wait to restore timer initiated after a failure on working path 118 a or 118 b may trigger a switchover from protection path 120 b to working path 118 b, and a switchover from protection path 120 a to working path 118 a). As a further example, a switchover may occur in response to a human-initiated action (e.g., a command to switchover issued by a network operator/administrator). All such events may be monitored by network elements 102 i and 102 j and all such monitoring may be synchronized between network elements 102 i and 102 j via synchronization connection 124. In addition, switchovers may be initiated by network element 102 i and/or network element 102 j in response to such monitored events, and such initiation of such switchovers may also be synchronized between network elements 102 i and 102 j via synchronization connection 124.

A technical advantage of network 100 c is that it provides a redundant solution to two L2 network domains that provide point-to-point L2 services.

FIG. 5 illustrates a block diagram of another example network 100 d including protection switching among multiple network elements (e.g., network elements 102 m and 102 n) interfaced between point-to-point network domains, in accordance with certain embodiments of the present disclosure. As shown in FIG. 5, network 100 d may include network element 102 l communicatively coupled to network element 102 m and network element 102 n via point-to-point network 122 c, and network elements 102 o and 102 p communicatively coupled to network element 102 m and network element 102 n via point-to-point network 122 d.

As described above, each of network elements 102 l-102 p may comprise any suitable system operable to transmit and receive traffic. Point-to-point network 122 c may be any network of one or more network elements (e.g., routers and switches) suitable to provide one or more point-to-point paths between network element 102 l and network element 102 m, and between network element 102 l and network element 102 n. Point-to-point network 122 b may be any network of one or more network elements (e.g., routers and switches) suitable to provide one or more point-to-point paths between network element 102 m and network element 102 o, between network element 102 m and network element 102 p, between network element 102 n and network element 102 o, and between network element 102 n and network element 102 p. Point-to-point network 122 c and 122 d depicted in FIG. 5 may be similar to point-to-point networks 122 depicted in FIGS. 1 and 3 and/or point-to-point networks 122 a and/or 112 b depicted in FIG. 4.

Network element 102 l may be multi-homed such that network elements 102 m and 102 n appear as a single network element to network element 102 l. Network elements 102 m and 102 n may be multi-homed such that network elements 102 o and 102 p appear as a single network element to each of network elements 102 m and 102 n. Network elements 102 o and 102 p may be multi-homed such that network elements 102 m and 102 n appear as a single network element to each of network elements 102 o and 102 p.

Network element 102 l may communicate with network elements 102 m and 102 n using linear protected switching. Accordingly, network element 102 l may be communicatively coupled to network elements 102 m and 102 n through a linearly protected switching connection. The linearly protected switching connection may comprise a working path 118 c and a protection path 120 c and may be similar to the linearly protected switching connections described above with respect to FIGS. 1, 3 and 4. Network element 102 l may be configured to perform linear protected switching between working path 118 c and protection path 120 c. For example, network element 102 l may be configured to perform protection switching between paths 118 c and 120 c in accordance with the G.8031 standard and may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching.

Furthermore, network element 102 m may communicate with network elements 102 o and 102 p using linear protected switching. Accordingly, network element 102 m may be communicatively coupled to network elements 102 o and 102 p through a linearly protected switching connection. The linearly protected switching connection may comprise a working path 118 d and a protection path 120 d and may be similar to the linearly protected switching connections described above with respect to FIGS. 1, 3 and 4. Network element 102 m may be configured to perform linear protected switching between working path 118 d and protection path 120 d. For example, network element 102 m may be configured to perform protection switching between paths 118 d and 120 d in accordance with the G.8031 standard and may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching.

In addition, network element 102 n may communicate with network elements 102 o and 102 p using linear protected switching. Accordingly, network element 102 n may be communicatively coupled to network elements 102 o and 102 p through a linearly protected switching connection. The linearly protected switching connection may comprise a working path 118 e and a protection path 120 e and may be similar to the linearly protected switching connections described above with respect to FIGS. 1, 3 and 4. Network element 102 n may be configured to perform linear protected switching between working path 118 e and protection path 120 e. For example, network element 102 n may be configured to perform protection switching between paths 118 e and 120 e in accordance with the G.8031 standard and may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching.

In addition, network elements 102 m and 102 n may be configured to implement protection switching between working path 118 c and protection path 120 c. For example, network elements 102 m and 102 n may be configured to perform protection switching between paths 118 c and 120 c in accordance with a modified multi-chassis version of the G.8031 standard and each may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching. In addition, network elements 102 m and 102 n may be communicatively coupled via a synchronization connection 124 a. Via synchronization connection 124 a, network elements 102 m and 102 n may communicate to each other such state information and switchover information. Synchronization connection 124 a depicted in FIG. 5 may be similar to synchronization connections 124 depicted in FIGS. 1, 3 and 4.

Similarly, network elements 102 o and 102 p may be configured to implement protection switching between working path 118 d and protection path 120 d and protection switching between working path 118 e and protection path 120 e. For example, network elements 102 o and 102 p may be configured to perform protection switching between paths 118 d and 120 d and between paths 118 e and 120 e in accordance with a modified multi-chassis version of the G.8031 standard and each may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching. In addition, network elements 102 o and 102 p may be communicatively coupled via a synchronization connection 124 b. Via synchronization connection 124 b, network elements 102 o and 102 p may communicate to each other such state information and switchover information. Synchronization connection 124 b depicted in FIG. 5 may be similar to synchronization connection 124 a and/or synchronization connections 124 depicted in FIGS. 1, 3 and 4.

In operation, paths 118 c and 120 c may provide connectivity for services between network element 102 l and network elements 102 m and 102 n (and ultimately, connectivity for services between network elements 102 l and 102 o and between network elements 102 l and 102 p). For a particular service, one of paths 118 c, 120 c may be designated as active, such that traffic for such service is communicated over such active path. In the event of a failure or other event, one or more of network elements 102 l, 102 m, and 102 n may cause a protection switch of the service from the active path to the other path (e.g., from working path 118 c to protection path 120 c). In certain embodiments, such protection switching may be implemented in accordance with the G.8031 standard.

Similarly, paths 118 d and 120 d may provide connectivity for services between network element 102 m and network elements 102 o and 102 p (and ultimately, connectivity for services between network elements 102 l and 102 o and between network elements 102 l and 102 p). For a particular service, one of paths 118 d, 120 d may be designated as active, such that traffic for such service is communicated over such active path. In the event of a failure or other event, one or more of network elements 102 m, 102 o, and 102 p may cause a protection switch of the service from the active path to the other path (e.g., from working path 118 d to protection path 120 d). In certain embodiments, such protection switching may be implemented in accordance with the G.8031 standard.

Moreover, paths 118 e and 120 e may provide connectivity for services between network element 102 n and network elements 102 o and 102 p (and ultimately, connectivity for services between network elements 102 l and 102 o and between network elements 102 l and 102 p). For a particular service, one of paths 118 e, 120 e may be designated as active, such that traffic for such service is communicated over such active path. In the event of a failure or other event, one or more of network elements 102 n, 102 o, and 102 p may cause a protection switch of the service from the active path to the other path (e.g., from working path 118 e to protection path 120 e). In certain embodiments, such protection switching may be implemented in accordance with the G.8031 standard.

In addition, during operation, each set of paths 118 c and 120 c, 118 d and 120 d, and 118 e and 120 e, may be configured in accordance with any suitable redundancy scheme. For example, a 1+1 protection scheme may be used in which traffic is replicated among paths making up a linearly protected switching connection, and an end point network element may determine which paths to select. As another example, a 1:1 scheme may be used in which all traffic is communicated over one path of a linearly protected switching connection and is protection switched to the other path of the linearly protected switching connection in response to an event.

Protection switching from one path to another may occur in response to any suitable event. For example, failure of any path of in a linearly protected switching connection may cause a switchover to the other path of the linearly protected switching connection (e.g., a failure in path 118 c may cause switchover to path 120 c, a failure in path 118 d may cause switchover to path 120 d, and a failure in path 118 e may cause switchover to path 120 e). Advantageously, in network 100 d a failure in one path may cause a switchover only in the linear protected switching connection including such path without leading to switchover of other linear protected switching connections, thus minimizing disruption. For example, a failure of path 118 d causing a switchover to path 120 d may have no effect on the protection switching status of paths 118 c and 120 c. In addition, a failure of path 118 c causing a switchover to path 120 c may cause traffic to be switched from the linear protected switching connection including paths 118 d and 120 d to the linear protected switching connection including paths 118 e and 120 e, but may have no effect on the protection switching statuses of the linear protected switching connection including paths 118 d, 120 d, 118 e, and 120 e.

As an additional example, a switchover may occur automatically after a failure condition has been remedied (e.g., a wait to restore timer initiated after a failure on working path 118 c may trigger a switchover from protection path 120 c to working path 118 c). As a further example, a switchover may occur in response to a human-initiated action (e.g., a command to switchover issued by a network operator/administrator).

All such events for the linear protected switching connection including paths 118 c and 120 c may be monitored by network elements 102 m and 102 n and all such monitoring may be synchronized between network elements 102 m and 102 n via synchronization connection 124 a. In addition, switchovers for the linear protected switching connection including paths 118 c and 120 c may be initiated by network element 102 m and/or network element 102 n in response to such monitored events, and such initiation of such switchovers may also be synchronized between network elements 102 m and 102 n via synchronization connection 124 a.

All such events for the linear protected switching connection including paths 118 d and 120 d and the linear protecting switching connection including paths 118 e and 120 e may be monitored by network elements 102 o and 102 p and all such monitoring may be synchronized between network elements 102 o and 102 p via synchronization connection 124 b. In addition, switchovers for the linear protected switching connection including paths 118 d and 120 d and the linear protected switching connection including paths 118 e and 120 e may be initiated by network element 102 o and/or network element 102 p in response to such monitored events, and such initiation of such switchovers may also be synchronized between network elements 102 o and 102 p via synchronization connection 124 b.

A technical advantage of network 100 d is that it provides a redundant solution to two L2 network domains that provide point-to-point L2 services using multiple levels of redundancy.

FIG. 6 illustrates a block diagram of an example network 110 e including protection switching among multiple network elements (e.g., network elements 102 r and 102 s) interfaced between a point-to-point network 122 and a multipoint-to-multipoint network 130, in accordance with certain embodiments of the present disclosure. As shown in FIG. 6, network 100 d may include network element 102 q communicatively coupled to network element 102 r and network element 102 s via point-to-point network 122, with network elements 102 r and 102 s communicatively coupled to multipoint-to-multipoint network 130.

As described above, each of network elements 102 q-102 s may comprise any suitable system operable to transmit and receive traffic. Point-to-point network 122 may be any network of one or more network elements (e.g., routers and switches) suitable to provide one or more point-to-point paths between network element 102 q and network element 102 r, and between network element 102 q and network element 102 s. Point-to-point network 122 depicted in FIG. 6 may be similar to point-to-point networks 122 depicted in FIGS. 1 and 3 and point-to-point networks 122 a and 122 b depicted in FIGS. 4 and 5.

Multipoint-to-multipoint network 130 may include a bridged network or similar network that uses flooding (e.g., broadcasting) and examination of source addresses in received packet headers to locate unknown devices in a network. Once a device has been located, its location may recorded in a table referenced by a unique address (e.g., Media Access Control (MAC) address) or the device so as to preclude the need for further broadcasting.

Network element 102 q may be multi-homed such that network elements 102 r and 102 s appear as a single network element to network element 102 q.

Network element 102 q may communicate with network elements 102 r and 102 s using linear protected switching. Accordingly, network element 102 q may be communicatively coupled to network elements 102 r and 102 s through a linearly protected switching connection. The linearly protected switching connection may comprise a working path 118 and a protection path 120 and may be similar to the linearly protected switching connections described above with respect to FIGS. 1, 3, 4, and 5. Network element 102 q may be configured to perform linear protected switching between working path 118 and protection path 120. For example, network element 102 q may be configured to perform protection switching between paths 118 and 120 in accordance with the G.8031 standard and may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching.

In addition, network elements 102 r and 102 s may be configured to implement protection switching between working path 118 and protection path 120. For example, network elements 102 r and 102 s may be configured to perform protection switching between paths 118 and 120 in accordance with a modified multi-chassis version of the G.8031 standard and each may thus maintain a G.8031 state machine for maintaining state information and switchover information for protection switching. In addition, network elements 102 r and 102 s may be communicatively coupled via a synchronization connection 124. Via synchronization connection 124, network elements 102 r and 102 s may communicate to each other such state information and switchover information. Synchronization connection 124 depicted in FIG. 6 may be similar to synchronization connections 124 depicted in FIGS. 1, 3 and 4 and synchronizations connections 124 a and 124 b depicted in FIG. 5.

In operation, paths 118 and 120 may provide connectivity for services between network element 102 q and network elements 102 r and 102 s (and ultimately, connectivity for services between network element 102 q and multipoint-to-multipoint network 130). For a particular service, one of paths 118, 120 may be designated as active, such that traffic for such service is communicated over such active path. In the event of a failure or other event, one or more of network elements 102 q, 102 r, and 102 s may cause a protection switch of the service from the active path to the other path (e.g., from working path 118 to protection path 120). In certain embodiments, such protection switching may be implemented in accordance with the G.8031 standard.

In addition, during operation paths 118 and 120 may be configured in accordance with any suitable redundancy scheme. For example, a 1+1 protection scheme may be used in which traffic is replicated among paths 118 and 120 and an end point network element may determine which path to select. As another example, a 1:1 scheme may be used in which all traffic is communicated over a single path (e.g., working path 118) and is protection switched to the other path (e.g., protection path 120) in response to an event.

Protection switching from one path to another may occur in response to any suitable event. For example, failure of path 118 may cause a switchover in to path 120. As an additional example, a switchover may occur automatically after a failure condition has been remedied (e.g., a wait to restore timer initiated after a failure on working path 118 may trigger a switchover from protection path 120 to working path 118). As a further example, a switchover may occur in response to a human-initiated action (e.g., a command to switchover issued by a network operator/administrator). All such events may be monitored by network elements 102 r and 102 s and all such monitoring may be synchronized between network elements 102 r and 102 s via synchronization connection 124. In addition, switchovers may be initiated by network element 102 r and/or network element 102 s in response to such monitored events, and such initiation of such switchovers may also be synchronized between network elements 102 r and 102 s via synchronization connection 124.

To support multipoint-to-multipoint technologies (e.g., bridging) in multipoint-to-multipoint network 130, network elements 102 r and 102 s may be configured to record unique addresses (e.g., MAC addresses) and locations of devices of multipoint-to-multipoint network 130. In addition, the record of unique addresses and locations of devices may be synchronized between network elements 102 r and 102 s via synchronization connection 124, so as to aid in re-convergence of multipoint-to-multipoint network 130 after switchover from one path 118, 120 to the other.

A technical advantage of network 100 e is that it provides a redundant solution to two L2 network domains (a point-to-point network and a multipoint-to-multipoint network) that provide L2 services.

A component of a network 100 (e.g., a network 100 a-100 e) may include an interface, logic, memory, and/or other suitable element. An interface receives input, sends output, processes the input and/or output, and/or performs other suitable operation. An interface may comprise hardware and/or software.

Logic performs the operations of the component, for example, executes instructions to generate output from input. Logic may include hardware, software, and/or other logic. Logic may be encoded in one or more tangible computer readable storage media and may perform operations when executed by a computer. Certain logic, such as a processor, may manage the operation of a component. Examples of a processor include one or more computers, one or more microprocessors, one or more applications, and/or other logic.

A memory stores information. A memory may comprise one or more tangible, computer-readable, and/or computer-executable storage medium. Examples of memory include computer memory (for example, Random Access Memory (RAM) or Read Only Memory (ROM)), mass storage media (for example, a hard disk), removable storage media (for example, a Compact Disk (CD) or a Digital Video Disk (DVD)), database and/or network storage (for example, a server), and/or other computer-readable medium.

Modifications, additions, or omissions may be made to networks 100 without departing from the scope of the invention. The components of networks 100 may be integrated or separated. Moreover, the operations of networks 100 may be performed by more, fewer, or other components. Additionally, operations of networks 100 may be performed using any suitable logic. As used in this document, “each” refers to each member of a set or each member of a subset of a set.

Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may be that alarm indication signals that typically originate from maintenance end points may be transmitted in the event that equipment upon which the maintenance end points have experienced a fault, thus reducing the occurrence of unnecessary alarms.

Although this disclosure has been described in terms of certain embodiments, alterations and permutations of the embodiments will be apparent to those skilled in the art. Accordingly, the above description of the embodiments does not constrain this disclosure. Other changes, substitutions, and alterations are possible without departing from the spirit and scope of this disclosure, as defined by the following claims. 

1. A method comprising: communicatively coupling a first network element to a second network element via a first path of a first point-to-point network; communicatively coupling the first network element to a third network element via a second path of the first-point-to-point network; communicatively coupling the second network element to a fourth network element via a first path of a second point-to-point network; communicatively coupling the third network element to the fourth network element via a second path of the second point-to-point network; and configuring the first path and the second path of the first point-to-point network as paths of a first linear protected switching connection and the first path and the second path of the second point-to-point network as paths of a second linear protected switching connection such that traffic associated with a service and communicated between the first network element and the fourth network element via the first path of the first point-to-point network and the first path of the second point-to-point network may be switched over to the second path of the first point-to-point network and the second path of the second point-to-point network in response to an event.
 2. A method according to claim 1, further comprising configuring the second network element, the third network element, and the fourth network element such that the second network element and the third network element appear as a single network element to the fourth network element.
 3. A method according to claim 1, further comprising configuring the first network element, the second network element, and the third network element such that the second network element and the third network element appear as a single network element to the first network element.
 4. A method according to claim 1, wherein at least one of: the first linear protected switching connection is configured in accordance with the ITU G.8031 standard with respect to the first network element; and the second linear protected switching connection is configured in accordance with the ITU G.8031 standard with respect to the fourth network element.
 5. A method according to claim 1, further comprising: communicatively coupling the second network element and the third network element via a synchronization connection; and configuring the second network element and the third network element to synchronize status information for the first linear protected switching connection and the second linear protected switching connection between the second network element and the third network element via the synchronization connection.
 6. A method according to claim 1, wherein the event comprises one of: a failure of the first path of the first point-to-point network; a failure of the second path of the first point-to-point network; a failure of the first path of second first point-to-point network; a failure of the second path of second first point-to-point network; a removal of a failure condition; and a human-initiated command to switchover.
 7. A method according to claim 1, further comprising configuring at least one of the first linear protected switching connection and the second linear protected switching connection for 1+1 redundancy protection.
 8. A method according to claim 1, further comprising configuring at least one of the first linear protected switching connection and the second linear protected switching connection for 1:1 redundancy protection.
 9. A method comprising: communicatively coupling a first network element to a second network element via a first path of a first point-to-point network; communicatively coupling the first network element to a third network element via a second path of the first-point-to-point network; communicatively coupling the second network element to a fourth network element via a first path of a second point-to-point network; communicatively coupling the second network element to a fifth network element via a second path of a second point-to-point network; communicatively coupling the third network element to a fourth network element via a third path of a second point-to-point network; communicatively coupling the third network element to a fifth network element via a fourth path of a second point-to-point network; configuring the first path and the second path of the first point-to-point network as paths of a first linear protected switching connection, the first path and the second path of the second point-to-point network as paths of a second linear protected switching connection, and the third path and the fourth path of the second point-to-point network as paths of a third linear protected switching connection, such that traffic associated with a service and communicated between the first network element and the fourth network element via the first path of the first point-to-point network and the second linear protected switching connection may be switched over to the second path of the first point-to-point network and the third linear protected switching connection in response to an event.
 10. A method according to claim 9, further comprising configuring the first path and the second path of the second point-to-point network such that traffic associated with a service and communicated via one of the first path of the second point-to-point network and the second path of the second point-to-point network may be switched over to the other of the first path of the second point-to-point network and the second path of the second point-to-point network in response to a second event.
 11. A method according to claim 9, further comprising configuring the third path and the fourth path of the second point-to-point network such that traffic associated with a service and communicated via one of the third path of the second point-to-point network and the fourth path of the second point-to-point network may be switched over to the other of the third path of the second point-to-point network and the fourth path of the second point-to-point network in response to a second event.
 12. A method according to claim 9, wherein at least one of: the first linear protected switching connection is configured in accordance with the ITU G.8031 standard with respect to the first network element; the second linear protected switching connection is configured in accordance with the ITU G.8031 standard with respect to the second network element; and the third linear protected switching connection is configured in accordance with the ITU G.8031 standard with respect to the third network element.
 13. A method according to claim 9, further comprising: communicatively coupling the second network element and the third network element via a synchronization connection; and configuring the second network element and the third network element to synchronize status information for the first linear protected switching connection via the synchronization connection.
 14. A method according to claim 13, further comprising configuring the second network element and the third network element to synchronize status information for the second linear protected switching connection and the third linear protected switching connection via the synchronization connection.
 15. A method according to claim 9, further comprising: communicatively coupling the fourth network element and the fifth network element via a synchronization connection; and configuring the fourth network element and the fifth network element to synchronize status information for the second linear protected switching connection and third linear protected switching connection via the synchronization connection.
 16. A method according to claim 9, wherein the event comprises one of: a failure of the first path of the first point-to-point network; a failure of the second path of the first point-to-point network; a removal of a failure condition; and a human-initiated command to switchover.
 17. A method according to claim 9, further comprising configuring at least one of the first linear protected switching connection and the second linear protected switching connection for 1+1 redundancy protection.
 18. A method according to claim 9, further comprising configuring at least one of the first linear protected switching connection and the second linear protected switching connection for 1:1 redundancy protection.
 19. A network element comprising: logic for communicatively coupling the network element to a second network element via a first path of a first point-to-point network; logic for communicatively coupling the network element and to a third network element via a synchronization connection; logic for communicatively coupling the network element to a fourth network element via a first path of a second point-to-point network; wherein: the second network element and the third network element are configured to be communicatively coupled via a second path of the first point-to-point network such that the first path and the second path of the first point-to-point network may be configured as paths of a first linear protected switching connection; the third network element and the fourth network element are configured to be communicatively coupled via a second path of the second point-to-point network such that the first path and the second path of the second point-to-point network may be configured as paths of a second linear protected switching connection; and traffic associated with a service and communicated between the second network element and the fourth network element via the first path of the first point-to-point network and the first path of the second point-to-point network may be switched over to the second path of the first point-to-point network and the second path of the second point-to-point network in response to an event; logic for configuring the network element and the third network element to synchronize status information for the first linear protected switching connection and the second linear protected switching connection between the network element and the third network element via the synchronization connection.
 20. A network element comprising: logic for communicatively coupling the network element to a second network element via a first path of a first point-to-point network; logic for communicatively coupling the network element and to a third network element via a synchronization connection; logic for communicatively coupling the network element to a fourth network element via a first path of the second point-to-point network; logic for communicatively coupling the network element to a fifth network element via a second path of a second point-to-point network such that the first path and the second path of the second point-to-point network may be configured as paths of a second linear protected switching connection; wherein: the second network element and the third network element are configured to be communicatively coupled via a second path of the first point-to-point network such that the first path and the second path of the first point-to-point network may be configured as paths of a first linear protected switching connection; the third network element and the fourth network element are configured to be communicatively coupled via a third path of the second point-to-point network; the third network element and the fifth network element are configured to be communicatively coupled via a fourth path of the second point-to-point network such that the third path and the fourth path of the second point-to-point network may be configured as paths of a third linear protected switching connection; and traffic associated with a service and communicated between the second network element and the fourth network element via the first path of the first point-to-point network and the second linear protected switching connection may be switched over to the second path of the first point-to-point network and the third linear protected switching connection in response to an event; and logic for configuring the network element and the third network element to synchronize status information for at least one of the first linear protected switching connection, the second linear protected switching connection, and the third linear protected switching connection via the synchronization connection. 